Pulse combustors are the subject of two papers by one of the present inventors, Abbott A. Putnam; one entitled "General Survey of Pulse Combustion," in Proceedings of the First International Symposium on Pulsating Combustion, Sept. 20-23, 1971, University of Sheffield S1 3JD, England, and the other entitled "A Review of Pulse-Combustor Technology" presented at a symposium on Pulse Combustor Technology for Heating Applications at Argonne National Laboratory, Nov. 29-30, 1979.
Other related information is contained in U.S. Pat. Nos. 2,515,644 Goddard; 2,525,782 Dunbar; 2,546,966 Bodine; 2,878,790 Paris; 2,911,957 Kumm; 2,998,705 Porter; 3,118,804 Melenric; 3,267,985 Kitchen; 3,323,304 Llobet; 3,365,880 Grebe; 3,498,063 Lockwood; 3,792,581 Handa, and 4,033,120 Kentfield.
A well designed pulse combustor exerts a powerful pumping action. Hence, a small heating unit can automatically ingest a very large volumetric flow of air (or other combustion sustaining gas). It can eject its combustion products with great turbulence and velocity, under substantially high pressure. The combustion products can thus be forced through relatively long, narrow, and tortuous passages in a heat exchanger. The turbulence contributes to an overall high efficiency of heat transfer and to a self-cleaning action on the exchanger surfaces. The temperature of the combustion products can be reduced enough to condense the water vapor and thereby recover the heat of vaporization. Because of the high resistance to flow through the pulse combustor and the heat exchanger, there is very little loss of heat by convection when the pulse combustor is turned off. With the low exhaust temperature, a plastic exhaust duct can be used. No flue, chimney, or draft hood is needed. These factors together with the small size of the unit required for a given heat output allow for great flexibility of installation, including retrofit to existing heaters and boilers, at reduced capital cost.
Despite the foregoing advantages, the development of heating systems using pulse combustors has been held back by the problems mainly of noise and vibration reduction. These problems have seemed to defy analytical solution and to admit of only expensive and time-consuming emperical solutions. However, the last few years have been marked by fuel shortages and skyrocketing fuel cost. Pulse combustor systems can provide fuel savings, and this provides a major incentive to concentrate substantial financial and technical resources on the solutions to these problems. In his first article, supra, Putnam has shown basically how the use of multiple pulse-combustor units with tuned elements can provide acoustic cancellation of noise components as a practical solution to the noise problem.
Because of environmental considerations and restrictions, there has been some concern with the possibility that the use of pulse combustors in heating units might increase the production of objectionable nitrogenous compounds, e.g. nitrogen oxides (NO.sub.x). However, in his first article Putnam has pointed out that a two-stage unit, using pulse combustors fired fuel-rich and with intercooling in the first stage, could be operated with reduced production of NO.sub.x. In the two-stage units, secondary air is added at the exit of the resonance tubes to burn the excess fuel. As is known, the backflow from pulsed combustors having aerodynamic valves can be used to pump the secondary air. In the pulse combustors, the bulk of the fuel-rich mixture can be burned very rapidly and the temperature of the incomplete combustion products can be substantially reduced very quickly in the resonance tubes. In the second stage, the secondary air is added to the fuel-rich products of combustion which have had some thermal energy removed. Combustion thereby continues and is completed. In the second stage, the relatively low temperature and the short combustion time suppress the formation of nitrogen oxides.
This desirable, low-No.sub.x -producing operation may not be achieved in many cases, however, where it is not possible to remove heat from the pulse combustors at a sufficiently rapid rate. This problem is particularly apt to be encountered in the operation of warm air heating units.